The goal of this study is to investigate the cellular and molecular mechanisms underlying spasticity, and establish the groundwork for future translational studies in the clinic. A majority of US Veterans with SCI experience clinically significant spasticity, which can disrupt rehabilitation and negatively impact quality-of-life, e.g., mobility, personal hygiene, intimate relationships (Holtz et al., 2017; Skold et al., 1999; Walter et al., 2002). Current spasticity management strategies are palliative, and fail to address the underlying cause. Available treatment options also carry high risk for adverse effects due to non-specific action or long-term use (Adams et al., 2005; Kheder et al., 2012). A major hurdle facing the development of better treatments for SCI- induced spasticity is a lack of mechanistic insight into how injury leads to disability. To advance an evidence-based investigation toward improving spasticity management, we will carry out experiments with two objectives: In Objective 1, we will implement conditional knockout studies to understand the mechanistic contribution of Rac1 specifically in motor neurons and astrocytes to spasticity after SCI. Our previous work demonstrates that pharmacological inhibitors can block Rac1-regulated dendritic spine remodeling in motor neurons and reduce spasticity (Bandaru et al., 2015; Zhao et al., 2016). However, our studies thus far have relied upon the use of a pharmacological Rac1-inhibitor, NSC23766, which precluded our ability to determine the drug?s direct action on neurons. It is also unclear why NSC23766 rendered only partial restoration of normal reflex output, and dose- limiting side effects have prevented longer-term treatment. Thus, to clarify the contribution of Rac1 signaling in neurons and astrocytes, we will use a 1) virally-mediated Cre-Lox system to knockout Rac1 expression in motor neurons, and 2) transgenic mice lacking Rac1 specifically in astrocytes. Astrocytes are integral to synaptic plasticity and maintain neuronal hyperexcitability, but have not been studied within the context of spasticity after SCI. We will use electrophysiological and behavioral tests to measure evoked H-reflex excitability and spasticity. To control for other changes in motor function, we will also monitor gross locomotor function. To assess dendritic spine dysgenesis associated with spasticity, and other anatomical changes, we will perform image analyses in ?cleared? spinal cord tissue. In Objective 2, to establish the groundwork for clinical translation, we will also assess the feasibility of two translationally-relevant approaches targeting the Rac1-pathway. Specifically, first we will assess the utility of a viral-based gene therapy ?platform? to knockdown Rac1 expression and alleviate spasticity. We have previously used viral-delivery of custom-made shRNA constructs to effectively target misexpressed proteins and modify neuropathic pain after injury or disease (Samad et al., 2013; Tan et al., 2015). In the second approach, we will determine the potential utility of ?repurposing? romidepsin, a clinically available drug to disrupt PAK1, a downstream effector linking Rac1 to dendritic spine reorganization (Hayashi et al., 2007). In summary, findings from this study could be expected to not only improve the mechanistic understanding of spasticity and advance the field toward clinical application, but also potentially extend beyond SCI, to conditions such as MS, TBI, stroke that are widely prevalent among US Veterans.